The invention relates to a combined service brake and spring brake cylinder for vehicle brake systems, having a service brake piston, which is arranged in the housing of the service brake cylinder, and a spring brake piston, which is arranged in the housing of the spring brake cylinder, can be actuated by at least one accumulator spring and has a spring brake piston rod that projects through a central bore of a partition between the service brake cylinder and the spring brake cylinder in such a way that it acts on the service brake piston. A first sealing assembly is located in the central bore of the partition and seals off the interior of the service brake cylinder from the interior of the spring brake cylinder. The radial outer circumferential surface of the spring brake piston carries an additional sealing assembly for sealing off a spring chamber that houses the accumulator spring from a spring brake chamber of the spring brake cylinder, to which air can be admitted and from which air can be released.
A combined service brake and spring brake cylinder of the above-described type is known from DE 40 11 739 A1, for example. In DE 40 11 739 A1, the spring brake piston rod is sealed off by way of a sealing ring (not described in detail) inserted into the central bore of the partition. Furthermore, the spring brake piston is also sealed off relative to a radially inner wall of the spring brake cylinder by way of a sealing ring arranged on the outer circumference of the piston.
In such combined service brake and spring brake cylinders, the spring brake piston actuates the service brake piston with its spring brake piston rod when the parking brake is applied. The piston, for its part, has a service brake piston rod which interacts with a braking mechanism of the vehicle brake, in particular a disc brake. A brake mechanism of this kind is described in EP 0 740 085 B1, for example, and has a pivoted lever, which is pivotally connected to the service brake piston rod and is connected securely in terms of rotation to an application shaft provided with a cam contour that leads to an axial relative motion of a brake caliper and of a brake pad of a disc brake when the application shaft is turned about the longitudinal axis thereof.
Since the pivoted lever pivotally connected to the service brake piston rod turns about the application shaft during this movement, a tilting moment about an axis perpendicular to the longitudinal axis is exerted on the service brake piston rod and hence on the service brake piston. Because the relatively large end face of the spring brake piston rod is supported flat against the service brake piston when the parking brake is applied, this tilting moment is therefore also transmitted at least in part to the spring brake piston rod and to the spring brake piston. Since the spring brake piston rod is guided in the central bore of the partition by one sealing assembly, and the spring brake piston is guided relative to the spring brake cylinder by the other sealing assembly, this tilting moment acts at least in part also on the sealing assemblies, and this can lead to damage to the sealing assemblies in the long term and hence to a diminution of the sealing effect.
Given this situation, it is an object of the present invention to provide a combined service brake and spring brake cylinder of the above-mentioned type such that the sealing assemblies have a longer life.
According to the invention, at least one of the sealing assemblies contains a guide ring that guides the spring brake piston rod in the bore of the partition, or the spring brake piston in the spring brake cylinder. The ring is composed of a more rigid material than that of at least one sealing element. The sealing element is composed of an elastomer that is connected to the guide ring by a primary forming method such as injection molding or transfer molding.
The material selected for the guide ring can then be sufficiently rigid to ensure that the ring can accept the tilting moment acting on the spring brake piston rod and on the spring brake piston for a prolonged period without damage and can divert it into the partition and into the spring brake cylinder, while the material selected for the at least one sealing element can be sufficiently flexible to ensure that it has favorable sealing properties and, in particular, has to perform essentially no guiding function. The selected tolerances between the guide ring and the radially inner wall of the central bore of the partition and/or the radially inner wall of the spring brake piston can then be relatively large since the guide ring has to fulfill essentially no sealing function. This has a favorable effect on manufacturing costs.
In addition, the durability of the sealing assembly also increases because the at least one sealing element performing the sealing function can be made sufficiently flexible to ensure that the tilting moment no longer has a negative effect on the life of the sealing assembly.
Another favorable aspect for the life of the sealing assembly is that the sealing element forms an integral body with the guide ring, where the joining forces have been produced by primary forming, i.e. by direct shaping from a molding compound or a liquid starting material, by inserting the guide ring as a finished component into a die, into which the elastomer material is then introduced under the action of pressure and heat (vulcanization), for example. This is because a connection of this kind produced by primary forming is generally very durable.
In particular, the guide ring is composed of a plastic, such as acetal. As a particularly preferred option, the guide ring has at least one recess with an undercut cross-section, through which the at least one sealing element projects in such a way as to engage behind. The undercut cross-section provides a positive connection in addition to the material connection between the guide ring and the sealing element that derives from the primary forming method, making it even stronger and more durable.
According to a further aspect, provision can be made for the at least one sealing portion of the sealing element to have a dovetail cross-section with two arms at the end remote from the guide ring, one arm forming a seal against the radially inner wall of the central bore of the partition or against the spring brake cylinder, and the other arm forming a seal against the spring brake piston rod or against the spring brake piston. This provides a very flexible seal that can be adapted easily to the surface of the sliding partner.
If the at least one sealing element is of symmetrical design with respect to the guide ring and has two sealing portions each projecting axially beyond the guide ring, there are two sealing surfaces arranged one behind the other or in series in the direction of motion, and this advantageously enhances the sealing effect.
Such a sealing geometry can be produced in a simple manner in the context of the primary forming process. For example, the guide ring may have a plurality of axial through apertures, in particular apertures distributed around the circumference, through which the elastomer material then passes from both sides during the primary forming process. This simultaneously results in a positive connection between the guide ring and the sealing element if the two sealing portions projecting axially from the guide ring have larger outside dimensions than the inside dimensions of the apertures, thereby resulting in the formation of undercut cross-sections.
As a particularly preferred embodiment, the guide ring is designed to have a flexibility in the radial direction such that at least one radially protruding projection on the guide ring can be latched with a recess in the radially inner wall of the central bore of the partition. This enables the guide ring carrying the sealing element to be installed easily in the bore.
Not the least significant feature is that at least one cavity can be formed between the guide ring and the at least one sealing element in order to hold lubricant.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.